Multiphase eutectic ceramic coatings

ABSTRACT

Articles are described that include a substrate having thereon a coating that has two or more non-oxide compounds. A first one of the two or more non-oxide compounds is a non-oxide ceramic compound, and the two or more non-oxide compounds are present in relative amounts with respect to each other to form a eutectic composition mixture. The coating includes at least two discrete phases arranged in an interpenetrating three-dimensional microstructure.

CROSS-REFERENCES TO RELATED APPLICATIONS

This patent application claims priority to U.S. Provisional Patent Application Ser. No. 61/257,429, filed Nov. 2, 2009, the disclosure of which is incorporated herein by reference in its entirety. This patent application is also a continuation-in-part of U.S. patent application Ser. No. 12/871,358, filed Aug. 30, 2010, the disclosure of which is also incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

The present invention relates generally to ceramics, and more particularly to high toughness multiphase eutectic ceramics.

Carbon/carbon (C/C) and ceramic matrix composite (CMC) materials have demonstrated high strength and durability in extreme environment applications involving elevated temperatures and oxidizing environments such as those encountered in spacecraft atmospheric entry and aerospace propulsion applications. However, these materials are nevertheless susceptible to erosion in such environments, which can adversely impact their performance. Ceramic coatings can help protect these materials from erosion; however, such coatings are themselves often susceptible to problems with hermeticity, adherence, and durability.

Accordingly, it is desirable to provide additional materials that can withstand high temperatures and also provide high strength, toughness, and thermal shock resistance.

SUMMARY OF THE INVENTION

In an exemplary embodiment of the invention, an article comprises a substrate having thereon a coating comprising two or more non-oxide compounds. A first one of the two or more non-oxide compounds is a non-oxide ceramic compound, and the two or more non-oxide compounds are present in relative amounts with respect to each other to form a eutectic composition mixture. The coating also comprises at least two discrete phases arranged in an interpenetrating three-dimensional microstructure.

In another exemplary embodiment of the invention, a method of manufacturing is provided, comprising:

preparing a mixture comprising two or more non-oxide compounds, a first one of said two or more non-oxide ceramic compounds being a non-oxide ceramic compound, the two or more non-oxide compounds being present in relative amounts with respect to each other to form a eutectic composition mixture;

heating the eutectic composition mixture to form a molten eutectic composition mixture;

cooling the molten eutectic composition mixture to solidify the molten eutectic composition mixture to form a solid material comprising at least two discrete phases arranged in an interpenetrating three-dimensional microstructure;

pulverizing the solid material to form a powder material comprising powder particles that comprise two discrete phases arranged in an interpenetrating three-dimensional microstructure; and

applying to a substrate a coating comprising said powder material and then sintering the coating, or applying to the substrate a coating using thermal spray or vapor deposition with a feedstock comprising said powder material.

In yet another exemplary embodiment of the invention, a method of manufacturing is provided, comprising:

preparing a powder mixture comprising two or more non-oxide compounds, a first one of said two or more non-oxide ceramic compounds being a non-oxide ceramic compound, the two or more non-oxide compounds being present in relative amounts with respect to each other to form a eutectic composition mixture;

forming a shaped article from the powder mixture;

sintering the shaped article;

optionally, machining the shaped article to produce a net shape article; and.

heating the surface of the article to melt and then resolidify a surface portion of the article, thereby providing said surface portion comprising two discrete phases arranged in an interpenetrating three-dimensional microstructure.

In still another exemplary embodiment of the invention, a method of manufacturing is provided, comprising:

preparing a powder mixture comprising two or more non-oxide compounds, a first one of said two or more non-oxide ceramic compounds being a non-oxide ceramic compound, the two or more non-oxide compounds being present in relative amounts with respect to each other to form a eutectic composition mixture;

applying a coating comprising the powder mixture to a substrate; and

heating the powder mixture to melt and then resolidify at least a portion of the coating, thereby providing a coating comprising two discrete phases arranged in an interpenetrating three-dimensional microstructure.

These and other features and benefits will become more apparent from the following description taken in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which:

FIG. 1 depicts a photomicrograph of an exemplary embodiment of the invention of a coating comprising two discrete phases arranged in an interpenetrating three-dimensional microstructure.

DETAILED DESCRIPTION

The invention will be described with reference to specific embodiments, without limiting the same.

Embodiments of the present invention provide simple, low-cost methods of manufacturing high strength, high toughness multiphase ceramic coatings with unique self-assembled microstructures. Embodiments of the present invention are directed to materials systems, processes for manufacturing materials, and processes for manufacturing components comprising these systems.

Embodiments of the present invention are applicable but not limited to structures for extreme environments such as propulsion and aerothermal systems. Examples include but are not limited to non-eroding or shape-stable nosetips and control surfaces for hypersonic or atmospheric re-entry vehicles, non-eroding nozzle materials for propulsion systems, erosion-resistant engine materials, structural insulators, damage-tolerant hypervelocity particulate impact structures, and the like.

A key to strength and fracture toughness in fiber or particulate reinforced ceramic composites is engineered interfaces with salient mechanical behavior that inhibits the formation and rapid propagation of cracks under applied stress. Embodiments of the present invention achieve fracture toughness in multiphase ceramics through the formation of eutectic microstructures, i.e. three-dimensional interpenetrating phases with a large number of interfaces and commensurately high interfacial surface area. As these microstructures are self-assembled, fabrication of polycrystalline structures of the same is relatively simple when compared to complex fiber reinforced ceramics. The three-dimensional interpenetrating microstructure is illustrated in FIG. 1, which is a micrograph of a material having 45 mole percent HfB₂ and 55 mole percent SiC that can be prepared by mixing HfB₂ powder with SiC in the above-mentioned eutectic composition ratio. The material is then cold pressed into pellet shapes, and heated above the eutectic melting point of 2347° C. using any known methodology such as arc heating, followed by cooling to solidify the material, and then pulverizing it into a powder. The powder may then be used as a feedstock for a vacuum plasma spray coating applied onto a substrate such as carbon/carbon or ceramic matrix composite. After the coating is cooled, the coating will exhibit a three-dimensional interpenetrating microstructure. A representative interpenetrating microstructure is depicted in the photomicrograph shown in FIG. 1. As seen in FIG. 1, the interpenetrating microstructure has two discrete homogeneous phases where each phase has a highly continuous lamellar structure intertwined with the highly continuous lamellar structure of the other phase. The interphase interfaces are nearly atomically abrupt, with no measurable diffusion between the constituents. Due to the differential properties of the two constituents and effects stemming from rapid cool-down from the molten phase, these eutectic microstructures also have unique residual stress fields that contribute to their mechanical behavior. In the embodiment shown in FIG. 1, the interpenetrating eutectic microstructure has a lamellar microstructure. In another exemplary embodiment, the interpenetrating eutectic microstructure can have a rod-shaped configuration.

This structure is readily distinguished from conventional composite microstructures, which typically do not exhibit an interpenetrating lamellar structure, and instead often exhibit randomly interspersed agglomerations of particles or crystal structures, or a continuous primary phase with discrete particles or crystals of a secondary phase dispersed therein. Even when composites are reinforced with oriented fibers, they still do not provide a three-dimensional interpenetrating microstructure.

In exemplary embodiments, non-oxide ceramic compounds may include borides, carbides, silicides, and/or nitrides in a compound with a less electronegative element, including but not limited to hafnium, zirconium, tantalum, titanium, vanadium, chromium, niobium, molybdenum, tungsten, and rhenium. In a more specific exemplary embodiment, the less electronegative element is hafnium, zirconium, or tantalum. Some elements may serve as either the more electronegative or the less electronegative element in a non-oxide ceramic. For example, boron may be the more electronegative element such as in HfB₂, ZrB₂, or TaB₂, or it may be the less electronegative element such as in B₄C. Similarly, silicon may be the less electronegative element as with SiC, or it may be the more electronegative element such as HfSi₂. In one exemplary embodiment, the non-oxide ceramic compound is HfB₂, HfC, ZrB₂, ZrC, TaB₂, or TaC.

The non-oxide ceramic compound may be mixed with one or more additional non-oxide compounds that will together form a eutectic composition mixture. In an exemplary embodiment, the one or more additional compounds are selected from the group consisting of non-oxide ceramics, carbon, or metals. In another exemplary embodiment, the one or more additional compounds are selected from the group consisting of non-oxide ceramics and carbon. In yet another exemplary embodiment, the one or more additional compounds are each a non-oxide ceramic. In a still further exemplary embodiment, the one or more additional compounds are each a carbide. Examples of compounds that may be mixed with the first non-oxide ceramic compound include but are not limited to SiC, B₄C, HfC, ZrC, or C. In one exemplary embodiment, the eutectic combination is a two-compound eutectic combination. Examples of eutectic combinations that may be used in exemplary embodiments of the invention include, but are not limited to HfB₂/SiC (45/55 mole % ratio), HfB₂/HfC (66/34 mol % ratio), HfB₂/B₄C (22/78 mole % ratio), HfB₂/C (62/38 mole % ratio), HfC/SiC (28/72 mole % ratio), ZrB₂/SiC (55/45 mole % ratio), ZrB₂/ZrC (68/32 mole % ratio), ZrB₂/B₄C (35/65 mole % ratio), ZrB₂/C (67/33 mole % ratio), or ZrC/SiC (42/58 mole % ratio), and in an exemplary embodiment, the first and second compounds make up one of these combinations. In another exemplary embodiment the first and second compounds are HfB₂/SiC (45/55 mole % ratio) or ZrB₂/SiC (55/45 mole % ratio). The mole percentage ratios described above may be varied by modest amounts, provided that it does not lead to the formation of dendrites in one of the phases thereby disrupting the interpenetrating microstructure, and the term “eutectic” as used herein is intended to accommodate such compositional variations the true eutectic composition that do not disrupt the interpenetrating microstructure.

In an exemplary embodiment, a material is prepared by first mixing powders (e.g., having particle sizes of less than 44 μm) of a first non-oxide ceramic compound with a second non-oxide compound in eutectic amounts with respect to one another. The powder mixture may be cold pressed or cold isostatic pressed into a convenient shape for handling, and an optional binder as known in the ceramic art may be used to help hold the shape for handling. The pellets may optionally be pre-sintered at temperatures up to 2000° C. for durations up to 12 hours for improved handleability. Alternatively, the powder mixture may simply be held in a mold or crucible capable of withstanding the melting temperature of the eutectic composition mixture. The mixture is then heated above its eutectic melting point, followed by cooling and re-solidification, which results in the formation of a polycrystalline multi-phase interpenetrating three-dimensional microstructure. Heating may be performed in a high-temperature furnace or cold-pressed shapes such as pellets may be arc-melted. Other heating options include laser heating, microwave heating, plasma arc heating, and the like. In an exemplary embodiment, the rate of cooling may be controlled to affect certain properties of the microstructure, such as the interphase spacing. In one exemplary embodiment, the interphase spacing is less than 1 μm.

Additives may be included with the eutectic composition mixture prior to melting in order to control properties such as crystal size and growth and interfacial characteristics, in small amounts (e.g., less than 5%), as is well-known in the art, provided that they do not disrupt the eutectic properties and formation of the multi-phase interpenetrating three-dimensional microstructure. Such additives may include, but are not limited to carbon additives, certain oxides, and the like known to be used, for example, sintering aids or phase stabilizers.

In an exemplary embodiment, after the eutectic material has been cooled and solidified, it is pulverized into a powder, such as by known processes such as ball milling dry or in a volatile solvent, grinding, crushing, jet milling, and the like. In further exemplary embodiments, the particle size of the powder may be any particle size known to be useful for ceramic molding. In a more specific exemplary embodiment, the particle size may be less than 44 p.m. This powder may then be used to form coatings by various manufacturing techniques known to be useful for ceramics.

In one exemplary embodiment, the coating may be formed by melt or vapor process application of the ceramic material onto a substrate. Melt or vapor process applications may utilize a feed stream comprising the above-described powder having a three-dimensional interpenetrating network, or a eutectic powder mixture that has not formed the interpenetrating network (i.e., the mixture of components has not been melted, solidified, and pulverized), or a sintered article such powders or powder mixtures. Vapor process application may be accomplished by vapor deposition techniques, such as evaporative deposition or pulsed laser deposition. Melt process application of the ceramic material onto a substrate may be accomplished with thermal spray techniques. For example, plasma spray involves introducing the ceramic feed stream into a plasma jet emanating from a plasma torch and directed toward the substrate. Flame spray may also be used where the powder feed stream is introduced into a high-temperature combustion flame directed toward the substrate. Another alternative is high velocity oxygen fuel spraying (HVOF), where a fuel such as acetylene and oxygen are streamed together toward the substrate and ignited, and the powder feed stream is introduced into the combustion stream. In one exemplary embodiment, a variant on plasma spray, known as vacuum plasma spray, may be used. In vacuum plasma spraying, the plasma spray is done under low pressure and/or inert gas to minimize any by-products and minimize any process-induced oxidation of the coating materials.

Another approach to formation of a non-oxide eutectic ceramic coating involves applying heat to the surface of an article. In one exemplary embodiment, the powder as described above having the three-dimensional interpenetrating microstructure (e.g., prepared by mixing different ceramic powders, melting, solidifying, and pulverizing) is coated onto a substrate using conventional coating techniques, such as electrostatic or other powder spray techniques, or liquid coating techniques of a slurry of the powder (e.g., spray coating, brush coating, roll coating). In such coating techniques, a polymeric binder may optionally be used. The precise type of polymeric binder is not consequential as long as it provides the necessary coating properties, as it will be pyrolyzed during subsequent sintering. After the coating is applied, any solvent may be removed (or it may be flashed off during a staged heating as temperature is ramped up toward a sintering temperature), and the coating may be sintered at temperatures of 1000° C. to 2500° C., or higher, or exposed to localized surface heating with a high heat flux source such as a laser.

In another exemplary embodiment, a eutectic powder mixture that has not formed a three dimensional interpenetrating microstructure (i.e., the mixture of components has not been melted, solidified and pulverized) may be applied as a coating (e.g., using powder spray or coated as a slurry) onto a substrate. After this coating is applied, it may be subjected to localized heating with a high heat flux source such as a laser. In this case, the heat applied should be sufficient to cause localized melting of the coated material, which will then form a three dimensional interpenetrating microstructure upon solidification.

In yet another exemplary embodiment, a eutectic powder mixture that has not formed a three dimensional interpenetrating microstructure (i.e., the mixture of components has not been melted, solidified and pulverized) may be formed into a shaped article, and then sintered. Sintering generally involves hot pressing or hot isostatic pressing at temperatures of from 1000° C. to 2500° C. and pressures of from 1 ksi to 10 ksi. In some embodiments, the powder may be mixed with a solvent to form a paste, which can then be inserted into a mold, followed by drying and then hot-pressing. In other embodiments, the powder may be mixed with a binder and a solvent (e.g., a polymeric binder such as polystyrene), followed by evaporation of the solvent, which may allow for alternative molding techniques such as injection molding to form a green ceramic part, followed by sintering at temperatures as described above. In such embodiments, the binder generally pyrolyzes during the sintering process. Other techniques for manufacturing ceramic parts are disclosed in, for example, Introduction to Ceramics, W. D. Kingery, H. K. Bowen, & D Uhlman, 2^(nd) ed., 1976, John Wiley & Sons, or Introduction to the Principles of Ceramic Processing, J. Reed, 1988, John Wiley & Sons, the disclosure of each of which is incorporated herein by reference in its entirety. The article can be fabricated as a net-shape or near net-shape component, utilizing a variety of powder consolidation and densification methods as is known in the art, and also described in the above-referenced Kingery et al. and Bowen references. Post consolidation machining processes (e.g., cutting, grinding, drilling, routing, and other machining techniques, using high-hardness machining tools like diamond-coated tools capable of machining the components) can also be used to form components with complex geometries and tight tolerances on final dimensions. The sintered article will generally not exhibit a three dimensional interpenetrating microstructure, so localized surface heating is then applied (e.g., with a high heat flux source such as a laser) to melt at least a portion of the surface thereof, followed by solidification and accompanying formation of the three dimensional interpenetrating microstructure in the area of the localized surface heating.

Engineered eutectic coatings in multiconstituent ceramic systems comprising the materials described herein may be capable of surviving extreme environments at high temperatures. Furthermore, exemplary embodiments described herein yield polycrystalline eutectic components of high strength and toughness, and eliminate the need for the complex processing associated with single crystal formation. While not being bound by any particular theory or explanation, this high strength is believed to be at least in part due to the unique capability of the polycrystalline multi-phase interpenetrating three-dimensional microstructure to terminate and/or deflect the propagation of stress-induced micro-cracks at the interphase boundaries.

The description of the invention is provided to enable any person skilled in the art to practice the various embodiments described herein. While the present invention has been particularly described with reference to the various figures and embodiments, it should be understood that these are for illustration purposes only and should not be taken as limiting the scope of the invention.

There may be many other ways to implement the invention. Various functions and elements described herein may be partitioned differently from those shown without departing from the spirit and scope of the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and generic principles defined herein may be applied to other embodiments. Thus, many changes and modifications may be made to the invention, by one having ordinary skill in the art, without departing from the spirit and scope of the invention.

A reference to an element in the singular is not intended to mean “one and only one” unless specifically stated, but rather “one or more.” The term “some” refers to one or more. Underlined and/or italicized headings and subheadings are used for convenience only, do not limit the invention, and are not referred to in connection with the interpretation of the description of the invention. All structural and functional equivalents to the elements of the various embodiments described throughout this disclosure that are known or later come to be known to those of ordinary skill in the art are expressly incorporated herein by reference and intended to be encompassed by the invention. Moreover, nothing disclosed herein is intended to be dedicated to the public regardless of whether such disclosure is explicitly recited in the above description.

While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description. 

The invention claimed is:
 1. An article comprising a substrate having thereon a coating comprising two or more non-oxide compounds, a first one of said two or more non-oxide compounds being a non-oxide ceramic compound, the two or more non-oxide compounds being present in relative amounts with respect to each other to form a eutectic composition mixture, the coating comprising at least two discrete phases, each having a lamellar structure, wherein the lamellar structure of a first phase is interpenetrating with the lamellar structure of a second phase, the lamellar structures being arranged in a three-dimensional microstructure that is interpenetrating in all three dimensions and contains randomly oriented lamellae.
 2. The article of claim 1, wherein each of said discrete phases comprises a different one of said two or more non-oxide compounds.
 3. The article of claim 1, wherein the two or more non-oxide compounds other than said first compound are each selected from the group consisting of non-oxide ceramics, carbon, and metals.
 4. The article of claim 1, wherein the coating comprises a first compound selected from the group consisting of HfB₂, HfC, ZrB₂, ZrC, TaB₂, and TaC, and a second compound selected from the group consisting of SiC, B₄C, HfC, ZrC, and C.
 5. The article of claim 4, wherein the eutectic composition mixture of the first and second compounds is selected from the group consisting of HfB₂/SiC (about 45/55 mole % ratio), HfB₂/HfC (about 66/34 mol % ratio), HfB₂/B₄C (about 22/78 mole % ratio), HfB₂/C (about 62/38 mole % ratio), HfC/SiC (about 28/72 mole % ratio), ZrB₂/SiC (about 55/45 mole % ratio), ZrB₂/ZrC (about 68/32 mole % ratio), ZrB₂/B₄C (about 35/65 mole % ratio), ZrB₂/C (about 67/33 mole % ratio), and ZrC/SiC (about 42/58 mole % ratio).
 6. The article of claim 5, wherein the eutectic composition mixture of the first and second non-oxide compounds is HfB₂/SiC (about 45/55 mole % ratio) or ZrB₂/SiC (about 55/45 mole % ratio).
 7. The article of claim 1, wherein the three-dimensional microstructure has an interphase spacing of less than 10 μm.
 8. The article of claim 1, wherein the three-dimensional microstructure has an interphase spacing of less than 1 μm.
 9. The article of claim 1, wherein the substrate is a carbon-carbon composite or a ceramic matrix composite.
 10. The article of claim 1, wherein the substrate comprises said two or more non-oxide compounds not arranged in said three-dimensional microstructure.
 11. A manufacturing method, comprising the steps of: preparing a mixture comprising two or more non-oxide compounds, a first one of said two or more non-oxide compounds being a non-oxide ceramic compound, the two or more non-oxide compounds being present in relative amounts with respect to each other to form a eutectic composition mixture; heating the eutectic composition mixture to form a molten eutectic composition mixture; cooling the molten eutectic composition mixture to solidify the molten eutectic composition mixture to form a solid material comprising at least two discrete phases, each having a lamellar structure, arranged in an interpenetrating three-dimensional microstructure and containing randomly oriented lamellae; pulverizing the solid material to form a powder material comprising powder particles that comprise the at least two discrete phases arranged in the interpenetrating three-dimensional microstructure; and applying to a substrate a coating comprising said powder material and then sintering the coating, or applying to the substrate a coating using thermal spray or vapor deposition with a feedstock comprising said powder material.
 12. The method of claim 11, wherein a coating comprising said powder material is applied to the substrate and then sintered.
 13. The method of claim 11, wherein a coating is applied to the substrate using thermal spray or vapor deposition with a feedstock comprising said powder material.
 14. A manufacturing method, comprising the steps of: preparing a powder mixture comprising two or more non-oxide compounds, a first one of said two or more non-oxide compounds being a non-oxide ceramic compound, the two or more non-oxide compounds being present in relative amounts with respect to each other to form a eutectic composition mixture; forming a shaped article from the powder mixture; sintering the shaped article; optionally, machining the shaped article to produce a net shape article; and heating the surface of the article to melt and then resolidify a surface portion of the article, thereby providing a resolidified surface portion comprising at least two discrete phases, each having a lamellar structure, arranged in an interpenetrating three-dimensional microstructure and containing randomly oriented lamellae.
 15. A manufacturing method, comprising the steps of: preparing a powder mixture comprising two or more non-oxide compounds, a first one of said two or more non-oxide compounds being a non-oxide ceramic compound, the two or more non-oxide compounds being present in relative amounts with respect to each other to form a eutectic composition mixture; applying a coating comprising the powder mixture to a substrate; and heating the powder mixture to melt and then resolidify at least a portion of the coating, thereby providing a resolidified coating comprising at least two discrete phases, each having a lamellar structure, arranged in an interpenetrating three-dimensional microstructure and containing randomly oriented lamellae.
 16. The method of claim 15, wherein the powder mixture is heated by using thermal spray or vapor deposition during application of the coating.
 17. The method of claim 15, wherein the powder mixture is heated to melt at least a portion of the coating after application of the coating. 